Light-emitting diode (LED) technology has advanced to the point where LEDs can be used as energy efficient replacements for conventional incandescent and fluorescent light sources. One application where LEDs have been employed is in ambient lighting systems using white and color (e.g., red, green and blue) LEDs. Like incandescent and fluorescent light sources, the average luminous flux of an LED's output is controlled by the average current through the device. Unlike incandescent and fluorescent light sources, however, LEDs can be switched on and off almost instantaneously. As a result, their luminous flux can be controlled by switching circuits that switch the device current between two current states to achieve a desired average current corresponding to a desired luminous flux. This approach can also be used to control the relative intensities of red, green and blue (RGB) LED sources (or any other set of colored LED sources) in ambient lighting systems that mix colored LEDs in different ratios to achieve a desired color.
FIG. 1A illustrates a conventional LED light source 100, which includes a pulsewidth modulator (PWM) 101, a switched current source 102 referenced to ground, and an LED 103 floating between a supply voltage Vp and the high impedance side of the switched current source. The PWM 101 uses an n-bit linear counter 104 to count repetitively from 0 to 2n−1 over a period T=2n/fclock. A pulsewidth register 105 holds a value between 0 and 2n−1, representative of a desired duty cycle of the switched current source 102. A comparator 106 compares the value of the linear counter 104 to the value in the pulsewidth register. When the output of the counter is below the value in the pulsewidth register, the output of the comparator is low. When the output of the counter is at or above the value in the pulsewidth register, the output of the comparator is high. As a result, the duty cycle of the current source, and the average intensity of the LED, can be controlled by changing the value in the pulsewidth register.
FIG. 1B illustrates an array of LED light sources, which may include different color LEDs (e.g., red, green and blue) in different intensity proportions to generate different colors in combination.
In LED lighting, the luminous flux output (intensity) of each LED at a given operating current decreases as the junction temperature of the LED increases. LED junction temperature can increase due to power dissipation in the LED and increases in ambient temperature. This effect, illustrated in the curves of FIG. 1C for three selected LEDs, can create both luminous flux errors and errors in color mixing because the magnitude of the effect is different for LEDs of different colors.
Another temperature effect in LEDs is a shift of the dominant wavelength of an LED as the junction temperature of the LED changes. Typically, the dominant wavelength increases as junction temperature increases, causing a red shift. This effect can cause additional color distortion independent of the luminous flux effects.
At any given operating current, the forward bias voltage of an LED is a function of the junction temperature of the LED. If the forward voltages of the LEDs in an illumination array are known, then the junction temperatures can be determined and the overall spectral output of the array (i.e., color and intensity) can be controlled and corrected for changes in the junction temperatures of the LEDs. However, measuring the forward voltage of the LEDs in the conventional configuration is difficult because the LEDs are floating above ground and have a high common-mode voltage. In the conventional configuration, the LED forward voltages are measured as floating differential voltages and have to be measured through level-shifting voltage dividers and differential amplifiers that add complexity and measurement error. Additionally, the voltage dividers can leak current from the LEDs to ground, reducing LED intensity at a given drive level or increasing current consumption at a given intensity level.
In conventional LED arrays, the PWM output frequency is fixed, and therefore the spectral content of the control signal is concentrated in the PWM fundamental frequency and its harmonics. This may cause electromagnetic radiation that is concentrated in a narrow frequency range that may interfere with the operation of other circuitry in the illumination system or the local electronic environment.